The significant and distinguishing feature of synchronous motors in contrast to induction motors is that they are doubly excited. Electrical energy is supplied to both the field and the windings. When this is done, torque can be developed at only one speed--the synchronous speed. At any other speed the average torque is zero.
In three phase synchronous motors, the frequency of each phase is synchronized with the speed of the motor, that is, for a given line frequency there is a given synchronous speed. When, for some reason, the speed of the motor is not synchronized to the line of frequency, the speed--torque curve of the motor is disrupted. This results in a large current being passed through the motor without developing torque, therefore resulting in high dissipation. Under this condition, it is necessary to stop the motor and go through a restart-up procedure. This restart-up procedure consists of slowly increasing the line speed to track the motor speed until the proper motor speed is obtained.
One way in which an unsynchronized pulse can be injected across one of the phase terminals of the synchronous motor is from a line transient. Line transients can introduce unwanted pulses in the switching circuits that drive the synchronous motor. These switching transients can cause unwanted motor perturbations that require the motor to be shut down and restart. Another way that the motor can become unsynchronized is too rapid an acceleration. This results when the input line frequency is increased faster than the motor speed resulting in a high "slew" rate. Synchronous motors can tolerate a small slew rate of 1 to 2% but any greater slew rate could damage the motor.
To detect the addition of transients or high acceleration, missing pulse detectors have been utilized. These missing pulse detectors are of the type RC555 manufactured by Raytheon Company and described on page 7-4 of the Linear Integrated Circuit Data Book (1978). The device is configured such that the timing cycle is continuously reset by an input pulse train. A change in frequency, or a missing pulse, allows the timing cycle to go to completion and change the output level. This circuit has disadvantages in that it only runs at a single frequency.
In view of the above problems, there exists a need for a missing pulse detector that is operable over a wide input frequency range and compensates for both transients and excessive acceleration.